1. Field of the Invention
This invention relates to cathode ray tube electron beam deflection control systems and more particularly to electronic circuits which provide increased beam deflection rates with a minimum increase of power dissipation while maintaining linear beam deflection operation.
2. Description of the Prior Art
Beam deflection systems used in the prior art for linear deflection across the screen of a cathode ray tube generally employ amplifiers, the input signal to which is representative of the desired beam deflection, to provide an amplified voltage to the deflection yoke of the cathode ray tube which causes a current to flow through the yoke and a sample resistor in series therewith. The yoke current flowing through the sample resistor produces a voltage waveform representative of the deflection current. The voltage developed across the sample resistor is then subtracted from the amplifier input signal, establishing an amplfied error signal. Another voltage is developed across the sample resistor as a result of the error signal amplification and is subtracted from the input signal, which further reduces the error signal. Reduction of the error signal results in a reduction in the yoke voltage which changes the yoke current to track the desired input voltage waveform. This feedback voltage is used to control the deflection current in a linear and accurate manner with any change in the input signal being transformed into an equivalent current change in the deflection yoke. Proper design of this system results in a transconductance amplfier which provides a yoke current that is directly proportional to the input voltage.
This technique has proven to be quite effective for deflection systems with moderate beam deflection rate requirements. When an increase in the input signal establishes a deflection rate not within the linear range of the system, the amplifier will saturate and the yoke current will increase at a rate determined by the L/R time constant, of the yoke and sample resistor, and the saturation voltage. The amplifier is then operating open loop and cannot control the output current until the feedback voltage achieves a level at which it can regain control. Previous solutions to this problem increased the power supply or lowered the inductance of the yoke, both of which increases the power dissipation of the system. Other solutions established mode selectable dual voltage operation. However, this approach is limited to operations in which the modes are periodic and easily definable. The present invention provides for automatic switching of the supply voltages when required and results in linear beam deflection over an increased range of beam deflection rates with a minimum increase in power requirements.